在Android中使用自動提示輸入，控制元件為AutoCompleteTextView型別物件autoText，當給autoText設定介面卡時，繼承自ListAdapter或者BaseAdapter，需要實現介面Filter，並且自定義實現Filter子類。

public class AutoCompleteAdapter extends BaseAdapter implements Filterable{

    private Context context;
    private ArrayFilter mFilter;
    private List<String> mObjects;//過濾後的item
    private final Object mLock = new Object();
    private int maxMatch = 100;//最多顯示多少個選項,負數表示全部

    public AutoCompleteAdapter(Context context) {
        this.context = context;
    }

    @Override
    public Filter getFilter() {
        // TODO Auto-generated method stub
        if (mFilter == null) {
            mFilter = new ArrayFilter();
        }
        return mFilter;
    }

    private class ArrayFilter extends Filter {

        @Override
        protected FilterResults performFiltering(CharSequence prefix) {
            // TODO Auto-generated method stub
            FilterResults results = new FilterResults();
            if (prefix == null || prefix.length() == 0) {
                synchronized (mLock) {
                    ArrayList<String> list = new ArrayList<String>();
                    results.values = list;
                    results.count = list.size();
                    return results;
                }
            } else {
                String prefixString = prefix.toString();
                
                final ArrayList<String> newValues = new ArrayList<String>();
                ...
                results.values = newValues;
                results.count = newValues.size();
            }
            return results;
        }

        @Override
        protected void publishResults(CharSequence constraint,
                                      FilterResults results) {
            // TODO Auto-generated method stub
            mObjects = (List<String>) results.values;
            if (results.count > 0) {
                notifyDataSetChanged();
            } else {
                notifyDataSetInvalidated();
            }
        }
    }

    @Override
    public int getCount() {
        // TODO Auto-generated method stub
        return mObjects.size();
    }

    @Override
    public Object getItem(int position) {
        // TODO Auto-generated method stub
        //此方法有誤，儘量不要使用
        return mObjects.get(position);
    }

    @Override
    public long getItemId(int position) {
        // TODO Auto-generated method stub
        return position;
    }

    @Override
    public View getView(final int position, View convertView, ViewGroup parent) {
        // TODO Auto-generated method stub
        ViewHolder holder = null;
        if (convertView == null) {
            holder = new ViewHolder();
            LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = inflater.inflate(R.layout.list_item_login_users, null);
            holder.tv = (TextView) convertView.findViewById(android.R.id.text1);
            convertView.setTag(holder);
        } else {
            holder = (ViewHolder) convertView.getTag();
        }

        holder.tv.setText(mObjects.get(position));
        return convertView;
    }

    class ViewHolder {
        TextView tv;
    }
 

        給autoText設定介面卡

autoText.setAdapter(new AutoCompleteAdapter(getApplicationContext()));
        這時，發現只有在輸入字元超過兩個字元時才會給出下拉提示列表。在AutoCompleteTextView原始碼中找到處理文字變更的方法：

void doAfterTextChanged() {
        if (mBlockCompletion) return;

        // if the list was open before the keystroke, but closed afterwards,
        // then something in the keystroke processing (an input filter perhaps)
        // called performCompletion() and we shouldn't do any more processing.
        if (DEBUG) Log.v(TAG, "after text changed: openBefore=" + mOpenBefore
                + " open=" + isPopupShowing());
        if (mOpenBefore && !isPopupShowing()) {
            return;
        }

        // the drop down is shown only when a minimum number of characters
        // was typed in the text view
        if (enoughToFilter()) {
            if (mFilter != null) {
                mPopupCanBeUpdated = true;
                performFiltering(getText(), mLastKeyCode);
            }
        } else {
            // drop down is automatically dismissed when enough characters
            // are deleted from the text view
            if (!mPopup.isDropDownAlwaysVisible()) {
                dismissDropDown();
            }
            if (mFilter != null) {
                mFilter.filter(null);
            }
        }
    }
 

        這個方法裡有一個if(enoughFilter())的判斷，為true時才對字元進行判斷，為false直接傳null給filter()方法；那個這個enoughFilter()是什麼呢？看下面程式碼：

    /**
     * Returns <code>true</code> if the amount of text in the field meets
     * or exceeds the {@link #getThreshold} requirement.  You can override
     * this to impose a different standard for when filtering will be
     * triggered.
     */
    public boolean enoughToFilter() {
        if (DEBUG) Log.v(TAG, "Enough to filter: len=" + getText().length()
                + " threshold=" + mThreshold);
        return getText().length() >= mThreshold;
    }
 

        這個方法，判斷的是變更後文字的長度跟mThreshold這個值的大小比較，mThreshold是一個private的整型變數；這個變數有個預設的初始值為2：

public AutoCompleteTextView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);

        ...

        TypedArray a =
            context.obtainStyledAttributes(
                attrs, com.android.internal.R.styleable.AutoCompleteTextView, defStyle, 0);

        mThreshold = a.getInt(
                R.styleable.AutoCompleteTextView_completionThreshold, 2);

        ...

        a.recycle();

        ...
    }

        看到這裡，就明白了，需要設定Threshold的初始值，這個初始可以在XML中設定，也可以在Java程式碼中設定。

android:completionThreshold="1"

autoText.setThreshold(1);